This invention relates to the art of geophysical prospecting in general and to the art of radioactivity well logging in particular. Specifically, the invention relates to such logging wherein radiation scattered by a borehole environment and surrounding earth formations is analyzed to provide indications of certain pre-selected borehole and earth formation characteristics.
Radioactivity well logging conventionally employs penetrating radiation of two varieties: gamma rays and neutrons. In many well logging techniques, a source of radiation and dual radiation detectors mounted within the logging sonde are used. These two detectors are longitudinally-spaced in a logging sonde. The detectors detect radiation emitted by the source and scattered back to the sonde by the borehole environment and surrounding earth formation. Four-detector dual porosity compensated neutron logging systems are also known in the prior art. One four-detector system is discussed in H. Scott, C. Flaum and H. Sherman, Dual Porosity CNL Count Rate Processing, SPE 11146 (paper presented at the 57th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, Sept. 26-29, 1982). This four detector system uses two longitudinally-spaced thermal neutron detectors and two longitudinally-spaced epithermal neutron detectors to detect scattered radiation. Each pair of detectors provides an indication of borehole and formation characteristics.
In the two-detector system, as disclosed in U.S. Pat. No. 4,297,575 issued to H. Smith et al and assigned to Haliburton Co., signals produced by the two radiation count rate detectors, one of which is closer to a source of radiation in a sonde than is the other, are comparatively analyzed to provide indications of certain wellbore characteristics, such as formation bulk density and porosity. Additionally, the detector signals may be interpreted to provide indications of borehole environment conditions such as casing and cement thickness, salinity, tool stand-off, mudcake thickness, mud weight, and variations in borehole diameter.
Two basic prior art techniques are used to process dual detector count rate data. These techniques are commonly referred to as the "ratio" and "spine/rib" methods. The ratio method utilizes the ratio of detector responses to determine the parameter of interest. If the logging tool or sonde is calibrated in a reference "standard" well, and if the count rates produced by the two detectors are affected by the same proportion in non-standard environmental conditions, the ratio of count rates will tend to cancel the adverse effects of the non-standard environmental conditions. This technique is used in dual thermal neutron porosity logging. If, however, non-standard environmental conditions vary the count rates in each detector by different proportions, as when variations in borehole diameter vary the detector count rates, the spine/rib method may be more effective in determining borehole and environmental characteristics. Spine/rib analysis may be performed by plotting values obtained from the respective radiation detectors operating in the non-standard condition on a graph of values obtained from the sonde operating in known reference standard boreholes. The data obtained from the reference standard is referred to as the "spine", whereas the effect of non-standard environmental conditions is reflected in spine-intersecting lines referred to as "ribs". The point of intersection of a rib with the spine provides an indication of a corrected logging datum, for example, formation porosity.
Difficulties may be encountered when the spine/rib method produces ambiguous results in some borehole environments and earth formations that are evaluated in a single pass of a logging sonde through a borehole.
Moreover, the ratio method is not the preferred technique in some borehole environments and earth formations because it cannot account for borehole environmental and earth formation effects that are non-proportional.
Accordingly, it is an object of the present invention to provide a well-logging apparatus and method which combines the advantages of the spine/rib and ratio techniques.
It is another object of the present invention to provide a well-logging apparatus and method which automatically and continuously selects an appropriate analytical technique in accordance with the count rate data provided by the radiation detectors.
It is still another object of the present invention to provide a well-logging apparatus and method that accurately and unambiquously evaluates borehole and earth formations in a single pass of a well-logging sonde through a borehole.
These and other objects and advantages of the present invention will become apparent from the detailed description that follows.